WHERE DO GEMS FORM AND WHERE ARE THEY FOUND ?
Gems form in many different environments in the Earth. We will examine the most common and important environments and formation processes in this lecture.
It is important to distinguish where gems are formed from where they are found.
Almost all gems are formed below the Earth's surface.
After rock is brought to the surface, gems may be released from the rock by weathering (some minerals dissolve, others are transformed to clay minerals, and some others survive unchanged).
The minerals that survive unchanged may be washed into streams, etc., where they are concentrated by river / ocean processes.
Gems retrieved from alluvial deposits are often rounded due to rolling around in rivers and oceans
Gems often have quite a high specific gravity (density) compared to other minerals so that they are easily trapped in depressions in stream beds. This causes them to become concentrated and makes it easier to mine them. Other valuable and durable things are also concentrated by these processes. Gold is a well known example.
In the following sections we will examine how gems form. We will start with examples at or near the Earth's surface and move down into the mantle.
(1) Water near the Earth's surface interacts with minerals and dissolves them. The ability of these solutions to maintain elements in solution varies with physical conditions. If the solution conditions change (for example if the solution cools or evaporates), minerals will precipitate. A similar, familiar processes is formation of salt crystals by evaporation of sea water.
The mineral that forms is determined by what the dissolved elements are. If the water has interacted with silica-rich rocks (e.g., sandstone), silica-rich minerals will form:
amethyst (quartz); agate ;
and the formation
of opal.
Of these, only opal is non-crystalline (ordered blobs of gel less than a micron in diameter).
Formation from water near the Earth's surface:
If the water has interacted with copper-rich rocks, copper minerals will form:
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This movie (112k) shows formation of agate, amethyst, opal, turquoise, and malachite/azurite. Note the importance of alternating wet periods (when solutions are dilute and can dissolve a large amount of silica, copper, etc.) and dry periods (when solutions evaporate and minerals precipitate).
Here is some additional expalantory text for this movie!
The formation of gems by hydrothermal processes is not dissimilar to formation of gems from water near the Earth's surface
The solutions involve rain water and/or water derived from cooling magma bodies< Gems crystallize from solution when it encounters open spaces such as cracks. As a result, 'veins' of minerals fill preexisting cracks.
Minerals such as beryl (e.g., emerald), tourmaline need unusual elements, and some of these, like beryllium (for beryl) or boron (for tourmaline) are derived from cooling molten rock (magma).
This process is illustrated in this movie (84k)
Here is some additional expalanation of this movie.
Pegmatites are unusual magma bodies.
As the main magma body cools, water originally present in low concentrations becomes concentrated in the molten rock because it does not get incorporated into most minerals that crystallize. Consequently, the last, uncrystallized fraction is water rich. It is also rich in other weird elements that also do not like to go into ordinary minerals.
When this water-rich magma (also rich in silica and unusual elements) is expelled in the final stages of crystallization of the magma, it solidifies to form a pegmatite.
The high water content of the magma makes it possible for the crystals to grow quickly, so pegmatite crystals are often large. Of course, this is important for gem specimens!
When the pegmatite magma is rich in beryllium, crystals of beryl form.
If magmas are rich in boron, tourmaline will crystallize.
You should note that beryllium and boron are extremely rare elements in most rocks and it is only because the above process efficiently concentrates these unusual elements that crystallization of boron and beryllium-rich minerals can occur.
This movie (40 k) shows formation of crystals such as emeralds and tourmaline in pegmatite bodies associated with cooling intrusive (magmatic) rocks
Here is some additional information about this movie
More information on pegmatites.
Some gems crystallize in magmas or in gas bubbles (holes) in volcanic rocks. Examples include: zircon, topaz, ruby, etc.
This movie (56 k) shows formation of crystals such as ruby or zircon (pink crystals) and topaz @in open cavities (e.g., holes made by gas bubbles) in volcanic rocks.
Metamorphic rocks are rocks changed by heat, pressure, and interaction with solutions. There are a number of types of metamorphic environments:
This
movie shows metamorphism of rocks resulting
from continent-continent collision associated with a subduction zone.
Note the formation of large crustals such as
garnet in the deformed, heated zone.
This movies illustrates the process of contact metamorphism . This is the process by which the minerals in rocks change in response to proximity to a hot intrusive body. For example, a limestone intruded by a magma undergoes significant change in crystal size, mineral content, and chemistry (due to addition of solutions released from the cooling magma). These rocks contain gems such as garnet.
Rocks such as kimberlites are erruptive volcanics that come from quite deep in the mantle and carry with them diamonds. Diamonds are made from carbon. The stable form of carbon at the Earth's surface is graphite. High pressures and temperatures are required to convert graphite to diamond. Thus, almost all diamonds formed about 100 miles below the Earth's surface. Dates suggest that their formation was restricted to in the first few billion years of Earth history.
Rarely, diamonds are formed in very high temperature and pressure metamorphic rocks.
This movie (68 k) shows that diamonds do not form in the kimberlite magma but are carried up to the surface by the magma.
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